A technique for evaluating the deterioration condition or remaining life of a rechargeable battery by analyzing the complex impedance of the battery has been proposed in the prior art. In this process, the battery state may be evaluated without breaking the battery. Thus, the battery may be reused when it is determined that the battery state is normal.
As an example of the process for analyzing complex impedance, Japanese Laid-Open Patent Publication No. 2000-299137 discloses a process for determining the initial activation degree and the deterioration degree of a rechargeable battery (refer to pp. 20-21 and FIG. 15). In this process, complex impedance is measured by applying an AC voltage of a frequency that is changed in a stepped manner to a rechargeable battery. Then, the real axis component and the imaginary axis component of the impedance are obtained from the measured values. When the values are plotted on a two-dimensional plane, a complex impedance line that includes a curved line and a straight line is obtained. Then, an approximation technique is conducted to obtain a diameter of a generally arc-like portion, which corresponds to the so-called charge transfer resistance region, of the complex impedance line. Then, it is determined whether or not the diameter is smaller than a predetermined threshold value. When the diameter of the generally arc-like portion of the complex impedance is smaller than the predetermined threshold value, it is determined that the initial activation degree of the battery is sufficient. When the diameter is larger than or equal to the predetermined value, it is determined that the initial activation degree is insufficient.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-299137